Wood possesses many desirable physical properties which make it an ideal material for many different applications. As compared to other building materials, wood is generally structurally strong, may be finished to a pleasing appearance and is easily shaped. Though a desirable material, wood has several physical properties which have limited its use in a number of different applications. These properties include the relative softness of wood, its typical non-uniform density and flammability.
Due to wood being an anisotropic material, i.e. the properties of wood are different in all three directions in which a tree grows, the specific gravity of wood may vary from 0.3 to over 1 depending on a number of factors. These factors include the particular species of wood, whether the wood is latewood (grown in the summer and early fall), summerwood (grown in the spring), heartwood or sapwood, the extractable content, mineral content, and other factors. Since the hardness of a wood surface is directly related to its specific gravity, the variability in specific gravity results in a wide variability in surface hardness. If a floor were made of oak, for example, the more dense latewood may have a specific gravity of approximately 0.7 whereas the less dense earlywood could have a specific gravity of approximately 0.5. The range of specific gravities is even wider in other species of wood.
Uniformity in hardness is especially desirable for wood that is used as a flooring material since soft spots in wood flooring are particularly vulnerable to permanent indentation resulting from the application of localized pressure. For example, a localized pressure of 100 pounds per square inch (psi) may result in an indentation in the earlywood of most wood species.
The pressure exerted on the surface of a wood floor varies with the size and weight of object in contact with the floor. For example, a person weighing 150 pounds wearing shoes with a 0.5 inch by 0.5 inch (normal high heal), may exert a pressure of up to 600 psi on the surface beneath the heel, The same 150 pound person wearing shoes with 0.25 inch by 0.25 inch (stiletto) heels may exert up to approximately 2400 psi on the surface beneath the heel, resulting in a permanent indentation. An exposed nail in a shoe heel with a much smaller surface area, can cause even more damage to a wood floor. For example, the same 150 pound person with an exposed nail having a diameter of 0.5 mm. can exert a specific pressure of 387,000 psi beneath the nail.
Several approaches have been taken in the past in attempts to harden wood surfaces. These attempts have included the application of surface coatings such as varnishes and impregnation of the wood with various materials. When hardness is achieved through a surface coating the ultimate hardness of this system is still controlled by the hardness of the wood under the coating. When indentation occurs on this surface, it may be more visible in a surface coated system than an uncoated surface due to the change in the refraction of the glossy coating. There have been attempts to harden wood surfaces by impregnating the wood with various materials such as methyl methacrylate with varying degrees of success. In the case of methyl methacrylate, however, the amount of polymer absorbed by the wood is limited. Moreover, since methyl methacrylate is a relatively volatile material, it may be lost through evaporation.
All of the components of wood, cellulose, hemicelluloses, lignin, and extractables undergo thermal degradation. If enough heat and oxygen are provided, wood will burn. Thus, in order to meet the demands of many applications, wood must be treated with a fire retardant.
Although various methods have been proposed to treat wood to promote hardness or to provide fire retardancy, in general the prior art methods present numerous deficiencies. In some cases the treatment does not give uniform hardness, in other cases the treatment does not provide a surface hard enough to be used for flooring and similar applications. Some treatments do not provide for staining in combination with treatment with hardening chemicals. Other treatments change the natural appearance of the wood. Where the treated wood is used as a veneer, some treatments permit bleed through of the adhesive used to adhere the veneer to the core material. In still other cases, a fire retardant can not be used in combination with the chemicals used to harden the wood, or the treatment process is too expensive, complicated and time consuming. In yet other cases, the chemicals used to treat the wood have relatively low vapor pressures, resulting in the loss of chemicals during the treatment process and the generation of potentially noxious fumes.
Thus, there exists a need for a practical and economical method of treating wood to improve its hardness that also permits treatment to improve fire retardancy and provides for staining of the wood surface.